The objective of this study is to quantitively measure the properties of the membrane current systems that regulate the firing rate of mammalian central neurons. To accomplish this we apply voltage clamp techniques to both in vivo cat motoneurons and to rat motoneurons grown in a tissue culture system. Because of better access to the cell membrane, these currents can be measured more accurately on the neurons grown in culture. But the development of the membrane properties of the neurons in culture may depend on the interaction of the neurons with other cells (both other neurons and glia) and possibly on the presence of specific growth factors. We hope to be able to determine which cell interactions are critical in the development of the different neuronal membrane properties and to determine whether non-nutrient growth factors might be involved. Eventually we hope to use this information about the neuronal membrane properties to be able to reconstruct the action potential firing properties of several different types of central neurons. Thus we hope to help in attaining a better understanding of the mechanisms regulating the firing rates of central neurons. BIBLIOGRAPHIC REFERENCES: Barrett, Ellen F. and Barrett, John N. (1976). Separation of Two Voltage-Sensitive Potassium Currents, and demonstration of a tetrodotoxin-resistant calcium current in frog motoneurones. T. Physiol. 255: 737-774.